Traditionally, a storage server is a networked computer that provides media and file access to other computers and computing devices, referred to herein as “clients.” The storage server may be accessible on a Wide-Area Network (WAN), a Local-Area Network (LAN), and/or share a dedicated point-to-point connection with another computing device.
Storage servers can be of several types, including a Network Attached Storage (NAS) server, a Storage Area Network (SAN) server, and an application-style server.
A NAS server comprehends directory structures. Requests to a NAS server generally specify a file path/filename and action such as create, read, write, append, etc. A NAS server converts the requests to one or more disk sector/block disk access transactions, accesses the disk (or other storage media), and performs the requested file-level transaction. Such a server maintains and utilizes a directory structure to locate the file starting locations, and a File Allocation Table to reconstruct segmented files and locate free sectors on a disk or other writeable media.
A SAN server generally receives direct requests for disk sector/block-based transactions, and may have no knowledge of a file system or relationships between data blocks. Although block requests can relate to physical block locations on a media device, more typically a SAN server supports logical blocks—thus allowing the server to make one physical disk drive appear as several logical drives, or several physical disk drives appear as one logical drive, as in a RAID (Redundant Array of Independent Disks) scheme.
An application-style server generally provides access to media in a media-specific format. For instance, an application-style server could provide access to streaming audio or video without the explicit knowledge of a filepath/filename by the client (who may not know the filename or have the authority to access a file directly), while reading the streaming media from files and encapsulating it in a streaming packet format.
Architecturally, most storage servers differ little from general-purpose computers (other than in some cases including more and/or faster than usual disk drives and disk bus controllers). FIG. 1 shows a block diagram for a typical storage server 20. A Central Processing Unit (CPU) 22, typically one or more microprocessors, operates the server according to stored program instructions. This type of processor is often called a von Neumann (VN) processor or machine, after its innovator, who proposed its execution style of processing sequential instructions.
CPU 22 connects through a Frontside Bus (FSB) 26 to a Memory Controller/Hub (MCH) 24, which is responsible for interfacing CPU 22 to other system components that store microprocessor program instructions and store, supply, and/or consume data. MCH 24 manages system memory 30 through memory bus 32. MCH 24 also communicates with PCI (Peripheral Component Interconnect) bridge 40 across hub bus 42 to move data between PCI-connected devices and either memory 30 or CPU 22.
Many different types of data source/sink devices can connect to server 20 through PCI bridge 40 and PCI bus 44. For storage server purposes, the two necessary devices are a network interface card (NIC) 50 and a media controller, such as ATA (AT Attachment) controller 60.
NIC 50 allows the server to attach directly to a Local Area Network (LAN) or other network, a Fibre Channel switch fabric, or other point-to-point connection to a client or another device that provides network visibility for the server. NIC 50 thereby provides a communication path for the server to receive data requests from clients and respond to those requests. The actual network physical communication protocol can be varied by inserting a different NIC, e.g., for whatever wired or wireless communication protocol is to be used, and loading a software device driver for that NIC onto CPU 22.
ATA controller 60 provides at least one ATA bus 62 for attaching media devices 64 such as hard disks, optical disks, and/or tape drives. Each ATA bus 62 allows either one or two media devices 64 to attach to storage server 20. Some servers may employ other controllers, such as a SATA (serial ATA) controller or a SCSI (Small Computer System Interface) host adapter.
In order to operate as a storage server, CPU 22 must execute a variety of software programs. With NAS, the two common formats are NFS (Networked File System) and CIFS (Common Internet File System); the former protocol is found primarily in UNIX environments, and the latter protocol in Microsoft operating system environments. For an NFS server, the following software processes help implement NFS: a network device driver for NIC 50; TCP/IP drivers; RPC (Remote Procedure Call) and XDR (External Data Representation) drivers to present data from TCP/IP to NFS server software; the NFS server software itself; a local file system; a VFS (Virtual File System) driver to interface the NFS server software with the local file system; data buffering software; and device drivers for the storage controller/host adapter. For each NFS data transaction, CPU 22 must execute a process for each of these software entities, switching context between them as required. To provide adequate performance, storage server 20 operates at relatively high speed and power, requiring forced air cooling and a substantial power supply to operate the microprocessor, chipset, network interface, memory, other peripherals, and cooling fans. The current state of the art, with a 1 Gbps (Gigabit per second) full duplex network connection, requires about 300 watts of power, occupies a volume of 800 cubic inches, and costs about $1500 without storage media.
Some vendors have attempted to build custom integrated circuit hardware explicitly for processing NAS requests. Design costs for such circuits generally cannot be recaptured in the fluid world of telecommunication protocols, where a specific complex circuit built to handle a narrow combination of specific protocols and storage interfaces may become rapidly obsolete.